Most oil and gas offshore production platforms presently in use are rigid structures having three or more rigid legs supporting one or more decks from which offshore oil wells are drilled and later produced. Supply boats are able to back down to these rigid structures and unload supplies or personnel by using any of various systems which have been developed. One such boat mooring system is described and illustrated in U.S. Pat. No. 4,359,011 which issued on Nov. 16, 1982 to Christopher J. Lyles.
Tension leg platforms, which are not rigid structures, have recently been designed and built for the development of oil fields in deep water, say, for example, in waters 2000 to 8000 feet deep. A tension leg platform is generally in the form of a large floating platform anchored at an offshore location by a vertical mooring system which may be enhanced by a lateral mooring system, if desired. The vertical mooring system is made up of a plurality of substantially vertical tendons which hang in tension between the buoyant platform on the surface of the ocean and the ocean floor where they are anchored to the ocean floor or to a rigid template. A lateral mooring system, if employed, may take the form of lateral catenary anchor lines extending from the platform outwardly and downwardly to anchors on the ocean floor. By using both anchoring systems, the floating platform is maintained in a substantially stationary position over wells to be subsequently drilled in the ocean floor.
A new method of using a tension leg platform to drill and maintain wells has been developed wherein the tendons and any production pipe lines extending between the platform and the ocean floor are sufficiently flexible in waters, say, 2500 feet deep, to allow the tendons and any vertical pipe lines to be flexed so that the floating platform may be moved laterally 400 feet or more to another fixed position. Thus, in drilling one well by this system, a centrally-located derrick on the drilling deck of the platform remains stationary over one drill site until the well is drilled, and/or completed, and/or shut in and abandoned. Subsequently, with the tendons of the tension leg platform anchored to the ocean floor, the floating platform supporting the flexible tendons is moved laterally from 50 feet to as much as 400 feet or more to a position directly over another well site where operations may be carried out.